sharing violation path exception

I am getting this error sometimes when I am trying to write a message to the xml file:

500 - Internal Server Error System.IO.IOException: Sharing violation on path /var/www/isoTopix/alt/importantMsg.xml at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) [0x00000] in :0 at System.IO.FileStream..ctor (System.String path, FileMode mode, FileAccess access, FileShare share) [0x00000] in ...

I have the following code that writes to the file

[WebMethod]
    public void AddToXMLFile (string param)
    {       
        XmlDocument doc = new XmlDocument ();

        DateTime today = DateTime.Today; 


            XmlNode rootNode = doc.CreateElement ("Scandiatransplant");
            XmlNode dateNode = doc.CreateElement ("Date");
            XmlNode importantMsg = doc.CreateElement ("Message");

            XmlAttribute msgId = doc.CreateAttribute ("id");
            msgId.Value = "msgID";

            dateNode.InnerText = today.ToString ();
            importantMsg.InnerText = param;
            importantMsg.Attributes.Append (msgId);

            rootNode.AppendChild (dateNode);
            rootNode.AppendChild (importantMsg);

            doc.AppendChild (rootNode);
            doc.Save (Global.sAppPath + "/alt/importantMsg.xml");

}

And the the code that reads from it

[WebMethod]
[ScriptMethod(UseHttpGet=true)]
public string GetMessage() {
    XmlTextReader reader = new XmlTextReader (Global.sAppPath + "/alt/importantMsg.xml");

    string message = null;

    while (reader.Read()) {
        if (reader.IsStartElement ()) {
            switch (reader.Name.ToString ()) {

            case "Message":
                message = reader.ReadString();
                break;
            }
        } 
    }

    return message;
}

I have seen a solution to a similar problem, where they are using a using statement like this:

    XmlWriterSettings settings = new XmlWriterSettings();
settings.CloseOutput = true;
using(XmlWriter writer = XmlWriter.Create(file.CreateText(), settings)){
//do work here
}

But I am not using a XMLWriter, so I can't do this.

I have tried doing this:

using(XmlDocument doc = new XmlDocument ())

But this does not work

Any ideas to what I can do to solve this issue?